Some interesting stats -- see the full article for more, and for some analysis/implications
Yahoo dominates e-mail with 88.4 million users in the United States in August, according to comScore. That is far more than Microsoft’s Windows Live Hotmail at 45.2 million and AOL at 44.8 million, not to mention Gmail at 26.0 million.
When you look at how much time people spend reading their e-mail, Yahoo mail users spend the most time (286 minutes a month), Gmail users the least (82 minutes), with AOL and Microsoft in the middle (229 and 204 minutes, respectively). When it comes to how many times users (defined as those who visited the site at least once in August) checked their e-mail, Google again was at the bottom, with 13.6 visits a month. Yahoo had the most loyal users, visiting 18.8 times that month, followed closely by AOL and then Microsoft.